The present invention relates generally to exposure apparatuses, and more particularly to a reflection type (cataoptric) projection optical system, an exposure apparatus, and a device fabricating method using the same. The reflection type projection optical system use ultraviolet (“UV”) and extreme ultraviolet (“EUV”) light to project and expose an object, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (“LCD”).
Along with recent demands for smaller and lower profile electronic devices, finer semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, the design rule for mask patterns has required that an image with a size of a line and space (“L & S”) of less than 0.1 μm be extensively formed and it is expected to require circuit patterns of less than 80 nm in the near future. The L & S denotes an image projected onto a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus as a typical exposure apparatus for fabricating semiconductor devices includes a projection optical system for projecting and exposing a pattern on a mask or a reticle (these terms are used interchangeably in the present application), onto a wafer. The resolution R of the projection exposure apparatus (i.e., a minimum size for a precise image transfer) can be defined using a light-source wavelength λ and the numerical aperture (“NA”) of the projection optical system as in the following equation:                     R        =                              k            1                    ×                      λ            NA                                              (        1        )            
As the shorter the wavelength becomes and the higher the NA increases, the better the resolution becomes. The recent trend has required that the resolution be a smaller value; however it is difficult to meet this requirement using only the increased NA, and the improved resolution expects use of a shortened wavelength. Exposure light sources have currently been in transition from KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm) to F2 excimer laser (with a wavelength of approximately 157 nm). Practical use of the EUV light is being promoted as a light source.
As a shorter wavelength of light limits usable glass materials for transmitting the light, it is advantageous for the projection optical system to use reflection elements, i.e., mirrors instead of using many refraction elements, i.e., lenses. No applicable glass materials have been proposed for the EUV light as exposure light, and a projection optical system could not include any lenses. It has thus been proposed to form a reflection type reduction projection optical system only with mirrors.
A mirror in a reflection type reduction projection optical system forms a multilayer film to enhance reflected light and increase reflectance, but the smaller number of mirrors is desirable to increase reflectance of the entire optical system. In addition, the projection optical system preferably uses the even number of mirrors to avoid mechanical interference between the mask and the wafer by arranging the mask and the wafer at opposite sides with respect to a pupil. Apparently, a two-mirror system is the minimum number, but it is difficult to maintain a high NA and good imaging performance only with two mirrors due to its limited freedom of design. Accordingly, Japanese Laid-Open Patent Application No. 2000-98228 has proposed a projection optical system including four mirrors.
In a reflection type projection optical system 1000 as the four-mirror system (M1-M4) as proposed in Japanese Laid-Open Patent Application No. 2000-98228, the incident light onto the mirror M1 and reflected light are close to each other near a stop AS as shown in FIG. 7. As a result, it becomes difficult to arrange one aperture stop and instead two non-circular stops become needed, disadvantageously causing the configuration complex. Here, FIG. 7 is a schematic sectional view of the conventional reflection type projection optical system 1000.
In addition, an exposure apparatus 2000 incorporated with the reflection type projection optical system 1000 arranges the mirror M2 closer to the object side than the stop AS, and a mechanism for holding the mirror M2 and a stop mechanism may possibly interfere with (or intercept) the illumination light incoming from a side direction of the projection optical system 1000, lowering the imaging performance. Here, FIG. 8 is a schematic structural view of the exposure apparatus 2000 incorporated with the reflection type projection optical system 1000.